Task app boot version dropdown always selected by default

[claudiahub]

Refers to spring-attic/spring-cloud-dataflow#5469

In the task launch view, the boot version dropdown stays to the default value despite the selection.

[claudiahub]

After debugging on this, it seems the API for the endpoint /tasks/executions/1?schemaTarget=boot2 does not have anymore the lastExecution property attribute which is necessary on our side in order to select the proper last selected boot version after the first has launched in the task.